Hi, I recently tried to change the folder and Multitask BG's on my 3GS, 4.1, LimeJb.
I changed the PNG files to what I wanted, then deleted the cache file.
It worked!
Then I tried to change the PNG file again to a different picture, then I headed over to delete the cache file again, but this time, my folders have gone black, and so has the Multitask bar, which also no longer shows any app multitasking, although they are.
If anyone has a fix, that would be great!
Although I do have an idea of my own...If someone could upload their cache file for me, then I replaced my cache file, with theirs, that might work...
I'm not too techy when it comes to cache and stuff, but I understand all the stuff about changing files, respringing, etc.

Cache file should replace itself. Try to delete the cache once again and respring. Also check to see if your new background file you replaced is the right size, file type( .png) and spelled correctly. Did you do it via winterboard or replaced the original file?